The present invention relates to a semiconductor device using flip-chip mounting and to a method of manufacturing the same. In particular, it relates to a semiconductor device having an rf transistor for use at frequencies ranging from the K-band to the millimeter-wave band and to a method of manufacturing the same.
As the remarkable technological progress has been achieved in the field of telecommunications in recent years, the frequency band used in communication devices has upwardly shifted from the microwave band to the millimeter-wave band. This entails a remarkable increase in the operating speed of a transistor used in the communication devices so that a device having a hetero-junction compound semiconductor transistor with a cut-off frequency over 100 GHz has been implemented lately. In such a communication device using radio frequencies ranging from the quasi-microwave band to the millimeter-wave band, however, a method of mounting a semiconductor chip composing a circuit as well as transistor characteristics presents problems. For example, a parasitic capacitance or a parasitic inductance is easily produced in most cases after the mounting step was completed. Since the effects exerted by the parasitic capacitance and the like on the communication devices become larger in proportion to the level of the frequency used in the communication devices, these parasitic reactance components should be reduced more as a higher frequency is used. In the communication devices using frequencies ranging from the microwave band to the millimeter-wave band, the size of a connecting element interposed between circuit components approaches the wavelength of a signal, so that careful consideration should be given to the size of the connecting element at the designing stage. Naturally, extreme precision and accuracy is required of circuit components including passive elements and lines.
To overcome the above problems and implement a low-cost, high-performance semiconductor integrated circuit operating at K-band and millimeter-wave frequencies and having a wide range of applications, a conventional technique termed MFIC (millimeter-wave flip-chip IC) has been proposed (The Institute of Electronics, Information and Communication Engineers, Autumn Conference '94 Term 39th). The technique is an IC (module) technique for reducing the parasitic effects by using a flip-chip bonding technique termed micro-bump bonding (hereinafter referred to as MBB), which enables a high-performance millimeter-wave IC to be implemented at low cost with some design flexibility, while taking advantage of the preciseness and manufacturability of the semiconductor fabrication process.
FIG. 32 is a cross-sectional view partially showing the structure of the MFIC, in which are shown: a substrate 1000 composed of Si or the like; a ground conductive film 1001 composed of an Au film formed on the main side of the substrate 1000; a dielectric film 1002 composed of a SiO.sub.2 film; and an interconnecting conductive film 1003 composed of a conductive material deposited and patterned on the dielectric film 1002. The interconnecting conductive film 1003, the ground conductive film 1001, and the dielectric film 1002 constitute a microstrip line. In the drawing are also shown: electrode pads 1004 included in the interconnecting conductive film 1003; a semiconductor chip 1008 with an embedded rf transistor composed of a compound semiconductor or the like; and electrode pads 1007 disposed on portions of the semiconductor chip 1008. The electrode pads 1007 are electrically connected to the electrode pads 1004 included in the interconnecting conductive film 1003 of the microstrip line via bumps (microbumps) 1006. A light-light setting insulation resin 105 is used to fix the semiconductor chip 1008 onto the substrate 1000 so that the connection provided by the bumps 1006 is enhanced by the contracting force of the light setting insulation resin 1005.
Next, the process of manufacturing the MFIC shown in FIG. 32 will be described with reference to FIGS. 33(a) to 33(e).
First, as shown in FIG. 33(a), the light setting insulation resin 1005 is supplied dropwise onto the substrate 1000 formed with the microstrip line. Next, as shown in FIG. 33(b), the bumps 1006 formed on the electrode pads 1007 of the semiconductor chip 1008 are aligned with the electrode pads 1004 included in the interconnecting conductive film 1003 on the substrate 1000 by using a camera or the like. Then, as shown in FIG. 33(c), the semiconductor chip 1008 is pressed by means of a pressing jig 1010 to extrude the light setting insulation resin 1005 from the space between the bumps 1006 and the electrode pads 1004, while the bumps 1006 are compressed and deformed to sink into the corresponding electrode pads 1004, thereby establishing connection thereto. Then, as shown in FIG. 33(d), the light setting insulation resin 1005 is cured under the radiation of an ultraviolet ray 1011 to fix the semiconductor chip 1008 onto the substrate 1000. During the curing process, the light setting insulation resin 1005 contracts to provide enhanced connection between the electrode pads 1007 and the electrode pads 1004. Then, as shown in FIG. 33(e), the pressing jig 1010 is removed after the curing process, thereby completing the mounting of the semiconductor chip 1008 on the substrate 1000.
By using the flip-chip mounting technique in accordance with the MBB method described above, the thickness of the bump 1006 can be reduced to th order of several micrometers or less so that the parasitic inductance induced by the intervening bumps 1006 is suppressed to an extremely low level (several picohenries), which renders the MFIC sufficiently usable at frequencies in the millimeter band. In a semiconductor device formed by flip-chip mounting employing a solder bump, the size of the bump is as large as about 50 .mu.m so that the bump functions as a distributed constant circuit or an inductor. On the other hand, in the MFIC formed by using flip-chip mounting in accordance with the MBB method, the thickness of the bump 1006 can be reduced to the order of several micrometers so that the function of the bump 1006 as an inductor is negligible. Moreover, since the microstrip line in the MFIC can be fabricated by using the semiconductor fabrication process, patterning can be performed with higher accuracy than in the case of a normal hybrid IC wherein interconnections are provided on a substrate of alumina or the like by employing a printing technique. Compared with an MMIC (millimeter-wave monolithic IC) similarly using the semiconductor fabrication process, the MFIC achieves a remarkable cost reduction since a passive circuit can be formed on a low-cost substrate of Si or the like, not on a substrate of a compound semiconductor.
Although the MFIC has numerous advantages as described above, it also has the following problems.
The first problem is a large loss in an rf signal when it passes through the microstrip line used in the conventional MFIC. Although a SiO.sub.2 film with a low dielectric constant is typically used to compose the dielectric film 1002 shown in FIG. 32, it is difficult to grow the SiO.sub.2 film with a large thickness over 10 .mu.m on the underlying ground conductive film composed of Au. In the case of forming a microstrip line with a characteristic impedance of 50.OMEGA., however, the line width W of the microstrip line and the thickness h of the SiO.sub.2 are determined to have a relationship substantially represented by W=2 h so that the line width W is inevitably reduced if the SiO.sub.2 film is thin. Consequently, the resistance of the line is increased, resulting in a conductor loss. Moreover, the dielectric loss or so-called tan.delta. of the SiO.sub.2 film is as large as about 0.03. The large conductor loss and large dielectric loss combine to increase a loss in the rf signal passing through the microstrip line.
If any material that may form a film with a large thickness over 10 .mu.m is used properly to compose the dielectric film, the line width can be increased and the conductor loss can be reduced, though the impedance remains the same. To form such a comparatively thick insulating film by a simple procedure, there is known a technique for forming an organic film of polyimide or the like that has been used for an interlayer insulating film of multilayer interconnections or a passivation film of an LSI. The technique enables the formation of a comparative thick dielectric film by a simple process including a spin-coating step and a baking step. By repeatedly performing these steps to stack multiple layers on the resulting dielectric film, a thicker film may be obtained. Moreover, since an organic film has a texture softer than that of an inorganic film, the substrate undergoes only a reduced stress even when the film thickness is increased so that the cracking or peeling off of the film due to a difference in coefficient of thermal expansion between the organic film and the substrate is easily prevented.
It is therefore a first object of the present invention to provide a semiconductor device with an embedded rf transistor wherein the dielectric film of the microstrip line is composed of an organic material particularly suitable for that purpose, which optimizes the impedance and prevents an increase in conductor loss.
However, if the organic film is used to compose the dielectric film of the microstrip line in the MFIC, the MFIC presents the second problem that the characteristics exhibited thereby may not be the same as assumed at the designing stage, though the conductor loss can be reduced. When the semiconductor chip 1008 is pressed against the substrate 1000 by means of the pressing jig in the step of mounting the semiconductor chip 1008 on the substrate 1000 via the bumps 1006, the dielectric film 1002 having a soft texture is deformed under the electrode pads 1004. Variations in the thickness of the dielectric film 1002 in the positions corresponding to the electrode pads 1004 cause the deviation of the line impedance from a value assumed at the designing stage, so that it becomes difficult to implement exactly the same performance assumed at the designing stage.
It is therefore a second object of the present invention to provide an MFIC having a microstrip line using a soft, thick dielectric film composed of an organic film or the like yet exhibiting exactly the same characteristic impedance assumed at the designing stage by providing a means for suppressing the deformation of the dielectric film during MBB mounting.
If the dielectric film is composed of a BCB film, the MFIC presents the third problem that the BCB film peels off the ground conductive film, the interconnecting conductive film peels off the BCB film, cracking occurs in the BCB film, or thermal deformation occurs during the manufacturing process. Close investigation has been conducted on the cause of the third problem, proving that unsatisfactory adhesion between the BCB film and the conductive film or the low heat resistance of the BCB film causes the third problem.
It is therefore a third object of the present invention to provide a highly reliable semiconductor device with excellent rf properties and a method of manufacturing the same by providing a means for compensating for the unsatisfactory adhesion and low heat resistance of the BCB film, while taking advantage of the excellent rf properties of the BCB film.